Many every day scenes contain a high range of intensities of light. When capturing an image of a scene, the range of intensities is often greater than the dynamic range of an image capture device. It is therefore often impossible to capture a sufficient tonal range in an image without suffering from excessive noise, particularly in dark areas of the image.
One method of overcoming this problem is to obtain multiple images of different exposures on an image capture device with low dynamic range and to combine these images to form a single higher dynamic range image. For example in U.S. Pat. No. 6,040,858 an expanded dynamic range is achieved by replacing an image signal with a proper signal in saturated or noise regions. The proper signal coming from a different exposure to the image signal.
Another method of increasing the dynamic range of an image obtained from a low dynamic range imaging device is to spatially vary the exposure of a scene. For example in U.S. Pat. No. 6,864,916 a high dynamic range image is produced using a low dynamic range sensor. The image of the scene is captured with an image sensor using a spatially varying exposure function. This can be done in a number of ways such as using an optical mask or by using an array of light sensing elements having spatially varying photosensitivies.
There are a number of problems with the prior art methods of increasing dynamic range. The combination of differently exposed images or image portions can result in loss of continuity of tone. Motion of an object in the scene between the different exposures can cause alignment problems when recombining the images. Further the recombination of the images can have substantial computational cost in terms of processing requirements.
Additionally spatially varying the exposure of a scene involves additional hardware and/or complex sensor modifications. These are expensive and can limit other aspects of the optical system such as the pixels size of the sensor or the volume of the optical system.